1. Field of the Invention
The present invention relates to an image sensor and a method of manufacturing the sensor, in particular, to an image sensor suitable for a contact image sensor, an image acquiring device and the like.
2. Description of the Related Art
As an image sensor, there is a type in which three kinds of light-receiving pixels that selectively receive lights in different wavelength ranges, for example, lights of three primary colors of red (R), green (G) and blue (B), respectively, are arrayed so as to be arranged on a substrate. In such an image sensor formed by arraying light-receiving pixels corresponding to the three primary colors of R, G and B, an image can be picked up in full color. However, the resolution of such an image sensor becomes one third as compared with the resolution of a monochromatic image sensor in which only the same kinds of light-receiving pixels are arrayed.
On the other hand, there is an image sensor formed by superposing image receiving pixels of R, G and B formed on separate substrates, respectively. Further, it has been proposed that an image sensor is formed by superposing image receiving pixels corresponding to two colors (for example, R and B) formed on both of the front and back surfaces of a single transparent substrate, and image receiving pixels corresponding to remaining one color (for example, G) formed on a separate substrate (for example, see, Japanese Patent Application Laid-Open No. 2007-67075). In this way, by superposing pixels corresponding to respective colors, an image pickup in full color can be attained and an improvement in resolution can be achieved. However, a gap equivalent to the thickness of the substrate (intermediate substrate) intervening between the light-receiving pixels is formed, resulting in a problem that images are apt to be blurred.
Further, it has been proposed that an image sensor in which a transmission and read-out circuit of a CMOS structure or a CCD structure is formed by a silicon (Si) semiconductor on a substrate, and light-receiving pixels corresponding to lights of R, G and B on the circuit are laminated via insulating layers, respectively (JP-A No. 2005-268609). In this case, it is considered that blurring of images can be prevented since a substrate having a larger thickness as compared with the insulating layer is not present between light-receiving pixels. However, it is difficult to manufacture such a multi-layer laminated image pickup device, since the electrode of each laminated light-receiving pixel is required to be connected to the transmission and read-out circuit formed on the substrate. Furthermore, since the transmission and read-out circuit formed on the substrate is made of a silicon semiconductor that requires a high temperature process, a flexible substrate made of plastic cannot be used.